Rainbow of Bubbles Test Tube - Section 6: Rainbow of Bubbles

pHantastic Chemical Reactions Day 1

Unit 6: Chemical Properties and Reactions
Lesson 6 of 9

Objective: Students will be able to identify evidence that the chemical properties of the reactants and products are different in a chemical reaction and identify common acids and bases by measuring their pH using various pH indicators.

Big Idea:
Students create a rainbow of bubbles in a test tube, write a mystery message, and more during labs on chemical changes! Students also try to find an explanation when the teacher models a new medical innovation by revealing a "bloody" hand print!

On Day 1 of this lesson, students review what occurs when an acid and base react and continue to work on identifying evidence that would indicate a reaction is a chemical or physical reaction. Students go through a series of fun lab stations that all provide evidence of chemical changes and particularly focus on changes in pH. On Day 2 of this lesson, the teacher places her hand in a mystery medical solution and touches a piece of goldenrod paper to reveal a bloody hand print! The students then engage in a discussion in order to determine the cause of this unexplained phenomena. Then, the students finish the pHantastic Chemical Reactions Lab Stations that they began in the previous lesson. Last, students conclude the lesson using a vocabulary strategy called "Shape Vocabulary".

This lesson is designed to connect to the following NGSS and Common Core Standards:

MS-PS1 - 2 Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.

During their discussions about the "bloody hand print", students construct, use, and/or present an oral argument supported by empirical evidence and scientific reasoning to support or refute an explanation or a model for a phenomenon or a solution to a problem. (SP7) In addition, in order to take part in this discussion, students gather, read, and synthesize information from multiple appropriate sources to develop their explanations. (SP8)When writing their lab responses, students construct a scientific explanation based on valid and reliable evidence obtained from sources (including the students’ own experiments). (SP6)

Crosscutting Concepts:

In developing their ideas of what occurs during a chemical reaction, students note patterns in their observations that can serve as evidence of changes of chemical properties such as gas production, color change, temperature change, and pH change. (Patterns)

Ask students "What are you going to learn today?". Students should respond with the essential question that is posted on the board and in their Chemistry Unit Plan which is "How do particles combine into new substances? What evidence can show how the physical and chemical properties of the substances change?".

Ask students to get out their Chemistry Unit Plan and self assess where they believe their level of mastery on Skill 5, "I can provide evidence to show if a reaction is a chemical or physical change." At this point, students have already done this during a previous lesson. This is an opportunity for them to make any changes if they feel that their level of mastery has grown. My students rank themselves on a scale of 1 to 4 (4 being mastery).

For a look at all the lessons that have led my students to this point and where we go from here check out the lessons in these units:

This unit is designed to answer the Essential Question, "How do particles combine into new substances? What evidence can show how the physical and chemical properties of the substances change?". It particularly focuses on types of matter, physical properties, phase changes, and factors that affect physical properties. This unit's purpose is so much more than just the content, however. It's focus is scientific literacy. It stresses group discussion, discourse and utilizing text references when engaging in argument. Students utilize reading, writing, and speaking strategies in order to develop scientific literacy. It's scientific literacy immersion!

This unit is also designed to answer the Essential Question, "How do particles combine into new substances? What evidence can show how the physical and chemical properties of the substances change?" This unit focuses on chemical properties and chemical reactions. Students analyze evidence and property changes that allow them to distinguish between chemical and physical reactions. In addition, students investigate the Law of Conservation of Mass as they look at how bonds are broken and formed in chemical reactions. This unit is full of hands on labs and station rotations that will engage any middle school student in chemistry!

Resources

In a previous lesson, students read the first two pages of the Acids and Bases Are Everywhere reading that is included in the resource section. In this lesson, they read the third page of the document. I ask students to "Talk to the Text" and work their way up the "Ladder of Discourse" as they interact with the text. Students document their thinking as they read in the margins of the text and try to reach real discourse as they interact with the text.

The levels of the "Ladder of Discourse" are "Tweets" (text to self connections), "Huh?'s"(questions or concepts they do not understand), "Found It" (finding answers to questions through context clues or finding science answers), and "Discourse" (combining ideas to think beyond the text).

For more background on "talking to the text" and the "Ladder of Discourse" check out the following lessons. These lessons include videos of me demonstrating these strategies and student work.

In the student work below, notice that the student "talked to the text" to show their thought process as they read.

After reading, as a class discuss the idea of what is occurring during an acid-base reaction. I like to show students how water and a salt are formed in a neutralization reaction. Without this discussion, I feel like students are simply memorizing the products and miss the conceptual understanding behind what is occurring. The following discussion connects to the students ideas of the structure of acids and bases as well as what occurs in a chemical reaction when bonds are broken and formed. Moreover, it begins the students realization of the Law of Conservation of Mass.

I write the following chemical equation on the board:

NaOH + HCl NaCl + H2O

At this point, students in my class are aware that in a chemical reaction bonds are broken and the elements and ions rearrange to form new bonds and molecules.

Teacher: "The bonds in the reactants are broken and we are left with the following ions."

I write each of the ions below the equation (Na+, OH-, H+, and Cl-).

Teacher: "What new bonds do you think would form if these ions rearranged and formed new molecules?."

Middle school students can recognize that positive and negative ions will "attract" and bond. Most of my students have that background knowledge that "opposites attract".

Student: "The H+ and OH- will bond."

Teacher: "What might form if I have two hydrogen and one oxygen?".

Student: "Water!".

Teacher: "Absolutely! Water, which is neutral! This is why mixing an acid and a base results in an overall pH that is more neutral. What else might be formed with what we have left?"

Student: "Salt!".

Teacher: "Ah! Yes, the products of a neutralization reaction are water and a salt! Do you notice that I keep saying a salt? In this reaction, the salt that you use daily is included. However, it is not always sodium hydroxide. There are many different kinds of salts! "A salt" is just a name for that group of molecules. In the labs today, you will be seeing a variety of acid base reactions."

Before beginning the labs, I also review the following criteria in lab writing and station work:

1. Scientists back up their claims with evidence!

a. For all of the questions that ask to provide evidence of a chemical change, students should also back up their claims with qualitative observations from the lab. For example, it is not enough to simply say, "gas production". Students should instead say something like, "There was gas production when bubbles formed in the bag."

b. For all questions asking to compare properties of the reactants to those of the products, I ask that the students include at least one physical property that changed and one chemical property that changes. I also emphasize that qualitative or quantitative evidence must be included. Qualitative observations could come from what they observed during the lab or they could choose to pull quantitative data from the pHantastic Chemical Reactions Properties Descriptions text. For example, in the pHantastic Chemical Reactions Properties Descriptions text for the Rainbow of Bubbles station, it states:

Reactants: Ammonia is a liquid that has a pH of 11 and a boiling point of -33˚C. Alka seltzer is a white solid that has a pH of about 9.

Products: Citric acid is a white powder with a pH of 2. Water has a boiling point of 100˚C and a pH of 7. Carbon dioxide is a colorless gas.

Instead of just writing "boiling point and pH" as their answer for a physical and chemical property that changed, they should write, "The boiling point of a reactant began as -33˚C while the product finished with a boiling point of 100˚C. The pH of the reactants were 11 and 9 while the pH of the product was 7 and 2."

2. All lab questions for a finished lab station must be completed before rotating to the next station. (I let my students rotate themselves as they finish as station work is something we do frequently. You may want to rotate the groups on a timer if this is not the case for your students.)

*The sections below highlight the set up and some quick pictures of each lab stations. For a closer look into student work and student discussions, see the lesson for Day 2 of pHantastic Chemical Reactions.

*Each station takes approximately 10 - 15 minutes. On Day 1, I only have the students complete 1 or 2 of these stations (depending on time). They will complete the rest on Day 2 of pHantastic Chemical Reactions.

Resources

1. Dip a Q-tip into the phenolphthalein. (Phenolphthalein is a base indicator. It turns pink in the presence of a base.) Use the wet swab to write a message on a white sheet of paper.

2. Spray the message using the bottle labeled “Ammonia”.

3. Spray the message using the bottle labeled “Vinegar”.

Teacher Tips:

In the spray bottle of the liquids, I mix about a 1/3 ammonia or vinegar to 2/3 water. Diluting the solution cuts down on the smell.

I put the phenolphthalein in the bottom of a beaker and add just a little water to make it like a "paste". This is what the Q-tips are dipped in.

It is crucial that there is a garbage can and paper towels at this station. IMMEDIATELY after each group/student finishes spraying the liquids, have them throw away the paper and wipe down the counter. Without this step, the odor in the room becomes very strong.

As mentioned earlier, it is important to wear goggles when spraying acids and bases.

If you have spent time with red cabbage juice as an indicator, students get the misconception that pink means "acid". Take the time to explain that phenolphthalein turns pink in the presence of a base. Students have the tendency to say that the message appears because "ammonia is an acid" when ammonia is really a base due to the fact that they do not realize that different pH indicators turn different colors than red cabbage juice.

If students are having a hard time determining whether vinegar and ammonia are acids or bases, I have them refer to their Household pHun! Lab document completed in a lesson previous to this.

In this activity the message appears because phenolphthalein is a base indicator and disappears because vinegar is an acid and it neutralizes the solution.

It is important that students have prior knowledge about red cabbage juice as an indicator. My students have already used cabbage juice in a prior lab (Household pHun). If your students are not aware of the colors that cabbage juice turns in the presence of acids or bases, I would provide them with a colored cabbage juice scale. The key idea here is that they recognize that mixing an acid and a base neutralizes the solution.

Resources (2)

Resources

After the first day, I stop the students after completing 2 lab stations so that they can complete this Formative Assessment as an exit ticket. Then, I sort them into stacks of similar learners that I meet with the next day. This way, I address misconceptions with chemical reactions and neutralization before they complete all of the lab stations. The lesson for Day 2 of this lab includes videos of me meeting with groups and the mini lesson/reteaching that I complete based on common misconceptions.

*The sections in this lesson highlight the set up and some quick pictures of each lab stations. For a closer look into student work and student discussions for each of the stations, see the lesson for Day 2 of pHantastic Chemical Reactions.

*Each station takes approximately 10 - 15 minutes. On Day 1, I only have the students complete 1 or 2 of these stations (depending on time). They will complete the rest on Day 2 of pHantastic Chemical Reactions.